With regard to a vehicular suspension arm that includes an arm main body made of a metal plate and a bush press-fit portion, provided at one end of the arm main body, into which a bush can be press-fitted, there has already been proposed, for example, an arrangement in which a pair of upper and lower inner peripheral flanges are formed by flanging inner peripheral parts of ring plate-shaped upper and lower end plates integrally connected respectively to one end of an upper plate and one end of a lower plate forming the arm main body so as to face each other inwardly, and inner peripheral faces of these inner peripheral flanges serve as bush press-fitting faces (ref. e.g. FIG. 4 of Patent Publication 1 below).
In this proposed arrangement, forward end faces of the pair of upper and lower inner peripheral flanges are made to abut against each other, and outer peripheral parts of the upper end plate and the lower end plate are bent inwardly so as to substantially conform to the inner peripheral flanges on the inside thereof, thus forming a pair of upper and lower outer peripheral flanges having forward end faces facing each other across a gap. It is therefore possible to impart to the bush press-fit portion the same level of rigidity and strength as that of a collar and, moreover, since there is no welded portion between the upper and lower end plates, it is unnecessary to employ a lap margin for welding, and there is the advantage that the arm end part shape can be made as small as possible.
A vehicular suspension arm, and in particular a suspension arm having a structure that includes an arm main body having one end connected to a vehicle body side and the other end connected to a wheel side, and a bush support member for press-fitting and supporting a bush connected via a connecting pin to a fork-shaped lower end part of a damper straddling a middle part of the arm main body with a gap, a drive shaft running through the gap, is conventionally known, but in a suspension arm having this structure, since it is necessary to ensure that there is a space on the upper side of the middle part of the arm main body, which serves as a damper support part, in order to avoid interfering with the drive shaft and a protective boot surrounding a joint thereof, and a space on the lower side thereof in order to avoid interfering with a wheel in a fully turned or full bump state, the space for installing the arm main body middle part is quite limited and, moreover, since a large damper support load acts on the arm main body middle part, it is necessary for it to be formed with high strength and high rigidity.
However, when a suspension arm is produced by forging or casting as shown in, for example, FIG. 28(a) in order to meet all these requirements, there are problems such as heavy weight and high cost. In order to solve these problems, an attempt has already been made to make a suspension arm by press-forming a metal plate, but such a suspension arm tends to have a large cross-sectional shape for an arm main body middle part, which serves as a damper support part, it becomes difficult to meet the recent demand for diversification in vehicle needs (for example, an increase in wheel diameter, an increase in engine output, an increase in the size of the drive shaft, etc.), and there are inconveniences such as the types of vehicle to which it is applied being greatly restricted.
Furthermore, as an improvement of the structure of the plate suspension arm, a structure has been proposed in which, as shown in for example, FIG. 28(b), a cylindrical straight sleeve is used as a bush support member for supporting a damper lower end part support bush on an arm main body middle part, and this sleeve is welded to opposite side walls of the arm main body middle part, but in this proposed structure, since it is necessary to ensure that there is a welding margin by making a collar end part protrude outward from the side wall of the arm main body middle part to a considerable extent (e.g. 7 mm), it is difficult to design the arm main body itself so as to have a sufficient cross-sectional width in the confined space around the arm main body middle part, and there is the problem that sufficient arm strength cannot be guaranteed, etc.
In order to solve this problem, for example, as shown in FIG. 8 of Patent Publication 2 below, an improved technique has been proposed in which a pair of left and right flange-equipped brackets (sleeve halves) are inserted from opposite sides of an arm main body, and the flanges are superimposed on and welded to the arm main body so as to give a sufficient cross-sectional width to the arm main body itself in a confined space around the arm main body middle part, thus satisfying the desired strength conditions.
Patent Publication 1: Japanese Patent Application Laid-open No. 2002-337525
Patent Publication 2: Japanese Patent Application Laid-open No. 2002-337524